Schizophrenia is a devastating neuropsychiatric disorder, affecting about 1.1% of the population over the age of 18. Human and mouse genetic studies have identified several schizophrenia susceptibility genes. Among them, neuregulin-1 (Nrg1), a pleiotropic signaling molecule, is confirmed as a risk gene for schizophrenia (Stefansson et al., 2002;Hall et al., 2006;Law et al., 2006). How functional changes in Nrg1 signaling lead to schizophrenia is an important research topic. Proteolytic cleavage of Nrg1 is required to release a functional fragment that will interact with its cognate receptors of the ErbB family to exert cell-cell signaling (Falls, 2003;Mei and Xiong, 2008). In our studies of BACE1, which was initially discovered as the -secretase for cleaving amyloid precursor protein to release A (Vassar et al., 1999;Yan et al., 1999;Hussain et al., 1999;Sinha et al., 1999;Lin et al., 2000), we have shown that BACE1 cleaves transmembrane Nrg1 to release a secreted EGF-domain- containing N-terminal fragment and to exert a signaling function (Hu et al., 2008;Fleck et al., 2013). Mice with deficiency in BACE1 exhibit altered Nrg1 signaling function and develop schizophrenia-like phenotypes (Savonenko et al., 2008). One intriguing question is whether enhancing Nrg1 activity in BACE1-null mice will ameliorate behaviors. Related to this question, we have recently generated a mouse model which expressed BACE1-cleaved Nrg1 N-terminal fragment (termed as Nrg1-ntf ) under the control of tetracycline (Tet) responsible element (Tet-Off promoter). We found that overexpression of Nrg1-ntf in transgenic mice (Tg- N1 /T mice) enhances Nrg1 signaling activity, as its downstream signaling molecules Akt and Erk are activated. However, contrary to our expectations, our functional study shows that Tg-N1 /T mice develop schizophrenia- like behaviors, which can be reversed if transgene expression is switched off. Results from our lab and others imply that abnormally hypo- or hyper-functional Nrg1 can lead to schizophrenia. Since the dys-regulated expression of schizophrenia susceptibility genes may affect normal brain development and lead to the gradual appearance of different symptoms at different ages (Piper et al., 2012;Powell, 2010;Rapoport et al., 2005), we aim to test our hypothesis that increased Nrg1 activity during early development, but not during adulthood, contributes to subsequent schizophrenia-like behaviors in the adult. To test our hypothesis in this study, we propose two specific aims. Aim 1: To determine whether increased Nrg1 activity in the adult has an impact on schizophrenia-like behaviors. Aim 2: To explore molecular mechanisms associated with schizophrenia-like behaviors in N1 /T transgenic mice.